Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

517
FXUS63 KLOT 251914
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
214 PM CDT Wed Mar 25 2026

.KEY MESSAGES...

- Elevated grass/brush fire threat continues this afternoon.

- Isolated to scattered thunderstorms expected south of I-80
  tonight, some may produce hail up to around 1" in diameter.

- Powerful cold front will move across the area Thursday with
  temps in the 70s/80s ahead it, falling sharply into the
  40s/50s behind it.

- Thunderstorms are likely to develop near the front Thursday
  afternoon, with the highest chances of thunderstorms near and
  south of I-80. There is a level 3 out of 5 severe threat in
  the region with large, possibly destructive hail and damaging
  winds the main threats.

&&

.DISCUSSION...
Issued at 215 PM CDT Wed Mar 25 2026

Today and Tonight...

An area of somewhat enhanced mid-level moisture (near 12-15 kft
agl) across Iowa is steadily translating eastward and will
arrive in our southwest locales through mid-late afternoon.
Water vapor loops and model analyses clearly reveal a low
amplitude wave centered near 850- 700 mb which will continue to
slide southeast across the forecast area through early evening.
Given the presence of steep mid-level lapse rates (at least in a
narrow zone aloft), there is at least a low potential for high
based convection to develop as this activity scoots across the
area. With significant sub-cloud dry air in place, if more
substantial shower (and isolated thunderstorm) activity
materializes, some strong wind gusts around 40 mph can`t be
ruled out, mainly across the southwest third of the CWA. Have
introduced some slight chances (mainly for showers) to cover
this potential through early evening.

More robust convective development will be possible later this
evening and into the early overnight period as a plume of
enhanced 925-850 mb moisture pushes east-northeastward out of
central Illinois. Intensifying isentropic ascent at the nose of
a southwesterly LLJ should allow at least widely scattered
elevated convection to blossom. Precisely where this occurs and
how much MUCAPE is available remains in question due to some
lingering model spread. The latest NAM has trended a bit farther
south and less robust with the degree of 850 mb moistening and
thus MUCAPE, more in line with the rest of the guidance suite.
In turn, have gently pushed the highest thunderstorm chances a
bit farther south closer to the Kankakee River vicinity and
points south. Given at least modest skinny CAPE profiles and
respectable deep layer shear with strong veering through the
column, some instances of marginally severe hail can`t be ruled
out with the strongest cores mainly across our far south and
southwest locales after about 9-10 PM, but this potential
appears limited in both scope and location at this time.

Any elevated convection should diminish and push east of the
region late tonight. This will set the stage for a very active
period of weather Thursday afternoon and evening.


Thursday...

One lingering question mark which will ultimately have a large
bearing on the precise location, timing, and general evolution
of the severe weather threat is the speed/location of a very
sharp backdoor cold front. Today`s guidance still remains
somewhat split on this, and in particular how quickly the front
initially rolls off the lake and across northern Illinois during
the late morning and early afternoon. The latest ECMWF remains
a reasonable middle ground solution, but a faster solution like
the 12z HRRR/WRFARW/GFS remains very much in play. As such, the
high temperature forecast across northern Illinois and lake-
adjacent portions of NW Indiana remains uncertain, with a very
large "bust" potential. If the front ends up slower,
temperatures in Chicagoland could surge into the upper 70s
before falling rapidly into the 40s.

By about 1 PM/early afternoon, there is decent agreement that
the front will be somewhere in the vicinity of a Rochelle to
Aurora to Valparaiso line and will continue to move southward at
a decent clip. To the south of the boundary, temperatures will
warm very quickly into the 80s (possibly mid to upper 80s in
spots). Deeper mixing may facilitate the development of stronger
southwesterly wind gusts and have boosted these a bit with this
forecast. The base of a warm EML will effectively cap this warm
sector to convection until at least 2-3 PM, if not even a bit
later than this. To the north of the front, markedly colder
conditions with gusty northeasterly winds will develop.
Increasing upper jet divergence and the arrival of a 700 mb
shortwave may allow initial elevated convective clusters to
develop across northern Illinois through mid afternoon.

Through mid-late afternoon, the cold front will continue to
press southward, likely in the vicinity of the Kankakee River
Valley by about 4 PM or so. Gradually-increasing large scale
ascent will eventually erode what lingering capping exists to
the south of the boundary and moistening/cooling atop the
frontal inversion, resulting in what should be generally
explosive thunderstorm development across the region around this
time period. The primary severe hazard during this time and
into Thursday evening will be from large, possibly VERY large
hail, particularly for elevated cells atop the shallow frontal
inversion. Deep layer shear is forecast to be extreme, with 1-7
km shear values approaching 60 to 80 knots. Additionally, modest
MUCAPE values and very strong storm- relative inflow winds (35+
knots) will be supportive of very wide/large updrafts,
facilitating large hail growth. Even though most storms will
remain elevated, there will be a localized damaging wind gust
threat (high res guidance depicts some degree of evenly- spaced
convective elements and associated 50+ knot gust output,
suggesting the potential for gravity wave damaging wind gusts).

Regarding the tornado potential: It remains unclear how many
storms will fire in the lingering free warm sector. The
HRRR/RAP/RRFS (which could be a bit overmixed) do not appear to
convect aggressively south of the front, possibly due to a
warm/dry PBL. However, if storms develop in this area and remain
discrete or semi-discrete, there would be a tornado potential
before the cold front rolls south of the forecast area. At this
time, the greatest tornado threat (based on the latest cold
front timing) appears to be relegated to areas south of about a
Streator to Rensselaer line, and this area would become
increasingly small with time. Even aggressively rightward-moving
supercells would also likely eventually get undercut by the
surging cold front (augmented by thunderstorm outflow to the
north).

Some localized training of west-east moving storms may result in
locally heavy rainfall, but this does not appear to be a set up
favoring widespread flash flooding.

Activity will eventually end as the main elevated instability
axis is shoved well south of us through the late evening. Right
now, it looks like precip will end prior to the arrival of sub-
freezing temperatures and do not have any snow mention in the
gridded forecast Thursday night.

Friday and beyond...

Broad cyclonic flow will develop across the Upper Great Lakes on
Friday as a sprawling mid 1030s mb high spreads across the
central CONUS. An embedded perturbation will drift across
northern Wisconsin and across northern lower Michigan on Friday,
and some guidance hints at the potential for some rain/snow
shower development during the afternoon across northern
Illinois. Boundary layer moisture looks pretty minimal in our
area, so have left a dry forecast at this time. Lingering
northwesterly breezes will quiet down Friday night and Saturday
morning as the surface high builds east of the Mississippi
River. After a cool start, Saturday looks like a decent day with
temperatures pushing back to around 50. Warmer and breezy
conditions are expected on Sunday as the surface pressure
gradient tightens in advance of the next system.

Forecast confidence diminishes for next week, but the large
scale pattern generally looks active in the region as broad
longwave troughing to our west will likely guide repeated
disturbances across the area. At the same time, an expansive
elevated mixed layer (EML) will continually expand and advect
eastward on increasing southwesterly mid-level flow. As a
result, shower and thunderstorm chances look like they`ll
persist intermittently through much of next week.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1227 PM CDT Wed Mar 25 2026

Key Aviation Messages for the current TAF period:

- Gusty southwest winds this afternoon to 30 knots

- Period of low-level wind shear this evening

- A strong cold front will push through tomorrow. Lower
  confidence on exact timing, but a wind shift from southwest to
  northeast with the front can be expected, with increasing
  gusts in excess of 20 knots in the afternoon

- Shower and thunder chances increase with the passage of the
  cold front.

Relatively quieter conditions in the near term compared to what
is expected tomorrow. Dry and VFR conditions are expected
through the rest of today. The main impacts will be from the
increasing gusty southwest winds. Gusts 20 to 25 knots can be
expected, but maximum gusts to 30 knots are possible this
afternoon. There is moderate confidence in wind gusts
diminishing around and after sunset. A strong low level jet is
expected to increase around and after sunset leading to low
level wind shear to be possible through around midnight.

Thursday will start with VFR conditions and southwest winds to
10 knots. However, a stout cold front will come racing southward
over northern Illinois tomorrow. Ground crews can expect a stark
temperature drop from the day`s high temperatures in the 60s
down into the 40s in under 2 hours or less. High res guidance
has been developing a weak pressure trough tomorrow morning
ahead of the front, allowing winds to switch from the southwest
to the northeast. Winds will then remain out of the northeast
and increase with gusts in excess of 20 knots with the passage
of the front in the afternoon. Scattered showers and the chances
for storms can be expected along and behind the front. Some of
these storms may be strong to even severe with the potential for
damaging hail and wind. While confidence was high enough to
include thunder mention in the TAF, confidence remains lower on
the overall timing of the wind switch and thunder arrival and
will need to be continually re-evaluated in later forecast
packages.

DK

&&

.FIRE WEATHER...
Issued at 215 PM CDT Wed Mar 25 2026

Southwesterly winds will gust up to 30 mph this afternoon.
Additionally, relative humidity values will settle into the 15
to 25 percent range, with the lowest readings occurring
generally near and east of I-55. This combination will result in
an increased threat for rapid grass and brush fire spread. After
7 PM, RH values will increase quickly and the strongest gusts
will ease. A special weather statement has been issued to
highlight the increased fire weather threat.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 8 PM CDT this evening for the IL
     and IN nearshore waters.

     Gale Warning from 5 PM Thursday to 7 AM CDT Friday for the IL
     and IN nearshore waters.

&&

$$

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